Grub Customizer is a software package that does exactly as the name would imply. It allows the user to customize different aspects of the grub boot menu – such as the order that entries appear in the list, how long grub waits before selecting a default system to boot to, etc.
Grub Customizer has been deleted from Ubuntu 22.04 repos. No explanation. Therefore, your instructions need to be updated.
I have been a longtime user of Grub Customizer and it has become problematic lately, not working correctly. Changing the grub boot screen colors and picture often fails. The basics do work seem to work, like boot order and time the screen appears, but I am seeing more and more people having trouble with Grub Customizer in the forums. I use Mint Mate where it is still in the repository, and Ubuntu Mate. Still it is easier than the command line to make the basic changes.,
Could you repair the grub file? If not, do it first.